Multistage electron multiplier



MULTISTAGE ELECTRON MULTIPLIER FiledDec. 15, 1937 Patented Mar. 5, 1940 UNITED STATES MULTISTAGE ELECTRON MULTIPLIER.

Werner Flechsig, Berlin-Charlottenburg, Germany, assignor to the firm Fernseh' Aktien- Gesellschaft, Zehlendorf, near Berlin, Germany Application December 15, 1937, Serial No. 180,001 In Germany December 5, 1936 6 Claims.

I This invention relates to electron multipliers, and particularly to electron multipliers producing a large output current. This invention deals with a known construction in which a plurality of substantially parallel electron-permeable or foil-like emitting electrodes are placed in back of each other in a direction perpendicular to the planes of the electrodes. In this construction, the primary electrons impact one side of the 10 electrode while the secondaries are drawn through the openings in the electrode towards the other side thereof.

It is known that a high current gain can be obtained with electron multipliers. However, as a consequence thereof, the load upon the last emitting stages increases corresponding to the increasing number of stages. Inasmuch as the emitting electrodes are not to be heated beyond a certain temperature for fear of impairing their capability to emit secondary electrons, a limit is set for the gain by the highest permissible load. If it is desired to obtain as high a current as possible in a certain stage without exceeding a certain load, it is advisible to choose the voltage as low as possible. However, as the gain of secondary electrons decreases with decreasing voltages and finally becomes smaller than unity, a minimum limit for the voltage is thereby determined.

In electron multipliers with successive meshworks, the distance between meshworks has usually been made several millimeters. Individual meshworks could readily be observed from the side, which is rather convenient in the forma- 85 tion process where it is important to watch the color of the electrode. If high currents are carried in such multipliers, space charge is developed between the electrodes which causes distortion in the amplification of alternating currents. Finally the current in one stage may become completely independent of the current in the preceding stage, thus also from the primary emission. Of course, the tube cannot be oper ated under such conditions. The most obvious remedy, via, to prevent space charge by application of a higher voltage, cannot be made use of because this would mean an increase in load. The invention provides an electron multiplier in which the described difiiculties are eliminated by decreasing to such extent the distance between electrodes in stages, in which space charge otherwise causes distortion of the original current, that space charge conditions no longer prevail. If this is done, the electrode spacing will become less than 2 millimeters.

In an arrangement of two electrodes between which a current flows, the current in one electrode first increases linearly with the emission. If the region of space charge is approached, a point is finally reached where the current at-' tains a certain .value independent of the emission of the source, the so-called space charge current. In this range emission of alternating magnitude produces constant current so that an originally undulating current produces a direct current of constant magnitude. This spacecharge current increases for a constant voltage with decreasing distance between the electrodes. According to the invention, the distance is so chosen that the space charge current corresponding to this distance is greater by at least two times than the mean current actually flowing between the electrodes.

For an arrangement of plane parallel electrodes at a distance of 2 mm., the space charge current at 30 volts is only about 10 ma. per square centimeter of area. Currents of between 4-5 ma. per square centimeter only can under such conditions be utilized without distortion. By decreasing the distance between electrodes to .05 mm., the space charge current can be increased sixteen times to a value of milliamperes and the useful mean current will accordingly be increased sixteen times to a value of 64-80 milliamperes.

In orderto obtain a satisfactory gain of secondaries, the voltage between emitting electrodes must be at least 30 volts. In order to obtain a high current in the last stages carrying a great load, it may be advisable to use higher voltages in the first stages where the load is still small and decrease the votages to approximately 30 volts in the final stages. Even better matching is obtained if the voltages between emitting electrodes in the last stages are decreased in steps from stage to stage.

While the last stages of the described tube may, for instance, be parallel and permeable to electrons, this is obviously unnecessary for the first stages. In this Way, a tube is gained in which the electrons emitted by the cathode first pass through any of the known multiplier stages, such as, for instance, an arrangement with a magnetic field according to Slepian, or an arrangement of the L or T type according to Zworykin, and later reach a meshwork or foil arrangement according to the invention.

Best matching to the space charge conditions at all points is obtained if the distance between emitting electrodes is decreased from stage to stage. The drawing shows this case as an embodiment of the invention. l is a thermionic cathode which may, of course, be replaced by a photoelectric cathode. 2 to H are emitting electrodes, while I2 indicates the final collecting electrode. In place of the plane arrangement shown, an arrangement of concentric cylinders surrounding each other, or any other suitable combination, can be used. The decrease in distance which is necessary from stage to stage is readily determined from the multiplication factor.

A greater distance can be provided between the last emitting electrode II and the anode l2 if the latter is capable of carrying a substantially higher load. In this case the space charge is preferably drawn away by means of a higher voltage. Simultaneously, the advantage of lower anode capacity is gained.

For high loads it is preferable to make the emitting electrodes of a material of greater temperature resistivity than the usual caesium electrodes.

Electrodes with barium as an emitter may be used which can be heated up to temperatures of about 400 centigrade without impairing the secondary-emission properties. The use of electrodes with a smaller secondary emission ratio than caesium for the last stages coincides with the intentions of the invention as it may often be advisable to operate the last stages at a lower gain so as not to obtain too unequal distances. Electrodes consisting of alloys may also be used, of which not only a superficial layer is emissive but the material proper. Such alloys contain, for instance, nickel, copper and chromium, whereto a metal of low work function as, for instance, barium or also caesium, is added. Such electrodes also have the advantage that a special forming process is unnecessary. These electrodes automatically obtain their most favorable emission conditions by the natural heating during the operation of the tube.

This invention is not limited to a tube for amplification of an alternating or undulating current, but can also be used for the generation of a constant high emission. In this case the current value may approach that of the spacecharge current. The electron multiplier, according to the invention, then represents the cathode of such a tube. If a thermionic cathode is used as the primary cathode of the electron multiplier, the saving in heater current is obvious.

This invention may also be utilized in tubes for generation of oscillations, such as, for instance, water-cooled transmitting tubes. Control is then preferably exerted at the primary cathode so that the control grid lies between the cathode and the emitting electrodes. In this case also a small cathode and low heater current can be used while heater currents of several hundred amperes have so far been required for transmitting tubes of high output. It may be understood that the multiplier, according to the invention, can also be combined with other electronic devices in one vacuum receptacle.

In a plane arrangement, or a practically equivalent cylindrical arrangement with comparatively large radii, the distance between electrodes must be decreased from stage to stage at a ra tio of 1:42 for constant voltage conditions between electrodes and for a gain of two per stage.

I claim:

1. An electron multiplier device comprising an anode, cathode and a plurality of electrode stages of amplification therebetween, having a graduated change in spacing between the electrode stages thereof for a substantially similar space charge condition between all stages at substantially the same output of said device, said anode being spaced farther from the nearest stage of amplification than the spacings between the electrode stages.

2. An electron multiplier device comprising an anode, cathode and a plurality of electrode stages of amplification therebetween, each electrode stage having a spacing with respect to an adjacent electrode stage for producing a space charge condition between adjacent electrode stages at substantially the same time, said anode being spaced farther from the nearest electrode stage than the spacing between the electrode stages.

3. An electron multiplier having a plurality of electrode stages of amplification with spacing between stages decreasing in value in the direction of increasing current, from approximately a spacing of two millimeters between the first and second electrode stages.

4. An electron multiplier device including a source of electrons and a collecting electrode and having a plurality of electrode stages of amplification with spacing between electrode stages decreasing in the ration of I 1: 2 in the direction of current increase through said device.

5. An electron multiplier device having a plurality of electrode stages of amplification and spacing between electrode stages decreasing in the direction of current increase through said device.

6. An electron multiplier device comprising an anode, a cathode and a plurality of electrode stages of amplification therebetween, each electrode stage having a spacing with respect to the preceding electrode stage greater than the spacing with respect to the succeeding electrode stage and less than two millimeters.

WERNER FLECI-ISIG. 

